The invention relates to a bimetal unit (1) for a circuit breaker, the bimetal unit (1) including a bimetal element (2) for releasing a trip mechanism of the circuit breaker. The bimetal element (2) is surrounded, in particular coaxially surrounded, by a ferrous ring (4) and a copper coil (5) wound around the ferrous ring (4). The ferrous ring (4) and copper coil (5) provide a compensation device which produces additional heat and, as a result, stronger bending to the bimetal element (2) when a current flows through the same bimetal element (2). The invention is useful to employ bimetal element (2) adapted for higher-rated current in a circuit breaker of lower-rated current as bimetal element (2) exhibits similar bending behavior as in higher-rated circuit breaker. This enhances calibration and reduces manufacturing costs. The invention also relates to a trip unit, a circuit breaker, a series of circuit breakers, and a method for calibrating a circuit breaker, each employing the bimetal unit (1).